The physical properties of edible oils and fats as obtained from agricultural sources do not necessarily correspond to the requirements of the food industry.
Consequently, several modification processes have been developed. In the hydrogenation process, a liquid oil is converted into a solid fat that can be used as hardstock in margarines and shortenings and at the same time increases its stability. In the interesterification process, physical properties of the material being interesterified are modified for instance by lowering its melting point and thereby avoiding a sticky mouthfeel. These processes modify an oil or oil blend and yield a single product. The fractionation process on the other hand separates the oil or fat in a higher melting stearin fraction and a lower melting olein fraction, each of which can yield further products by subsequent fractionation. Accordingly, the range of oil and fat products that can be produced by fractionation is very wide.
Various fractionation processes have been developed for edible oils and fats. Solvent fractionation processes use solvents such as acetone, nitropropane or hexane, but since these solvents are inflammable, their use requires an explosion-proof plant, which is expensive. Further expense is incurred by the removal of the solvent from the various fractions by distillation and by solvent loss. Accordingly, the solvent fractionation process is only used for the production of high-value specialities. There is also the detergent fractionation process but as a result of the improvements in the dry fractionation process, the detergent fractionation process can be regarded as superseded.
In dry fractionation, it is customary to heat the fat to be fractionated to about 10° C. above its melting point to erase crystal memory. The fat is then cooled slowly to below its melting point, whereupon crystals are formed and grow. When a sufficient degree of crystallisation has been attained, the crystal slurry is separated by filtration into a filter cake (the stearin), and a filtrate (the olein). Two types of crystallisation process are used. There is the process in which the melt is dispensed in trays and is not agitated during cooling. Such a process has been disclosed in EP 1 028 159A and can be advantageously used for oils such as palm kernel oil. The other type that is used for palm oil, anhydrous milk fat and various other oils, fats and butters employs large crystallisation vessels that are fitted with heat exchangers and an agitator.
Both types have in common that the filtration efficiency determines the yield of both fractions and the properties of the stearin. If the residual olein content of the filter cake is high, the stearin yield is high but the stearin properties are less extreme. Since in palm oil fractionation, the olein has a higher economic value than the palm stearin and the stearin economic value hardly depends on its properties, it is advantageous to aim for maximum filtration efficiency. This can be attained by using a membrane filter press in a batch process as disclosed in U.S. Pat. No. 5,198,123. A continuous filtration process employing a conical sieve centrifuge fitted with a co-rotating scroll has been disclosed in U.S. Pat. No. 4,542,036.
With the increasing production of palm oil, dry fractionation processes have become very important. Palm olein is a valuable cooking oil, palm oil mid fractions being used in confectionery applications, and palm stearin is used more and more as a component in the interesterification reaction mixtures used for the production of trans-free hardstocks for margarines and shortenings. Often these dry fractionation processes are integrated in palm oil refineries so that they can share the utilities and infrastructure. These refinery processes, such as degumming, bleaching and physical refining, are all continuous processes and differ in this respect from current fractionation processes, which are invariably batch processes.
Continuous fractionation processes have been developed starting from solvent fractionation processes. U.S. Pat. No. 4,127,597 discloses a process for fractionating tallow into three distinct fractions, a hard, high-melting solid fraction, a plastic solid having physical and thermal properties similar to those of cocoa butter, and a liquid oil fraction, comprising: (a) dissolving the tallow in a suitable solvent, the ratio of solvent to tallow being sufficient to solubilize the tallow and to effect a fractionation at a crystallizable ratio of solute concentration and temperature; (b) feeding, continuously, the solution to one or more crystallizers; (c) circulating the solution through the crystallizers at a first preselected steady state crystallization temperature range; (d) limiting the nominal residence time of said solution in the crystallizers at said first steady state crystallization temperature to a maximum of ten minutes; (e) crystallizing out a hard, high-melting solid thereby forming a circulating crystallized stream; (f) withdrawing continuously part of said crystallized stream at said first preselected steady state crystallization temperature to obtain crystallized hard, high-melting solid and filtrate; (g) recirculating, continuously, at said first preselected steady state crystallization temperature, the crystallized stream not withdrawn in step (f) together with aforesaid continuously fed solubilized tallow; (h) repeating steps (f) and (g) until all of the solubilised tallow is fed to the crystallizers and all of said crystallized stream is withdrawn from the crystallizers; (i) circulating the filtrate from the aforesaid first crystallization through said crystallizers at a second preselected steady state crystallization temperature range; (j) limiting the nominal residence time of said filtrate solution in the crystallizers at the steady state crystallization temperature to a maximum of 10 minutes; (k) crystallizing out a plastic solid having physical and thermal properties similar to those of cocoa butter thereby forming a circulating crystallized stream; (l) withdrawing continuously part of said crystallized stream at said second preselected steady state crystallization temperature to obtain crystallized plastic solid and filtrate; (m) recirculating, continuously, at said second preselected steady state crystallization temperature, the crystallized stream not withdrawn in step (l); (n) repeating steps (l) and (m) until all of said crystallized stream is withdrawn from the crystallizers; and (o) removing the solvent from the filtrate from the aforesaid second crystallization to obtain a liquid oil fraction. According to U.S. Pat. No. 4,594,259, suitable confectionery fats can be obtained by continuous fractionation of palm oil when using an acetone/fat ratio of about 5:1 to about 8:1 and employing two or more fractionation stages.
U.S. Pat. No. 4,839,191 discloses a process for the solvent fractionation of fats into at least two fractions including a first high melting glyceride fraction and a second fraction that is an oil at temperatures above 10° C., the process comprising the steps of: (a) dissolving the fat in a solvent which is a binary azeotropic solvent mixture, the solvent ratio being from 1.5 to 8.0 ml of solvent per gram of fat; (b) crystallizing the solution from step (a) at 10° C.-15° C.; (c) separately collecting a solvent phase and the precipitate formed in step (b); (d) extracting the precipitate of step (c) by contacting with fresh solvent cooled to about 2° C. below the temperature of step (b) using at least about 8% of the original volume of solvent; (e) collecting a solvent phase and a precipitate from step (d), which precipitate is a hard fat fraction having a melting point above 40° C.; and (f) combining the solvent phases from step (c) and step (e) and eliminating solvent therefrom to provide an oil fraction which is liquid above 10° C. This process can be performed either as a batch or as a continuous process.
The favouring of the use of solvents is quite understandable, since fats crystallise much faster from a solvent such as acetone than from the melt. In addition, the solvent dilutes the olein present in the filter cake so that for a given filtration efficiency, the stearin contains less olein resulting in its properties being less affected by olein than in the absence of the solvent. Moreover, the solvent fractionation processes listed above date from before the development of the current, efficient filtration systems employing for instance a membrane filter press.
Apart from these technological reasons, there are also physico-chemical ones. The fractional crystallisation of fats from a melt is a very complex process, because fats are mixtures of many different triacylglycerol molecules. Accordingly, the fat crystals formed during fractionation are mixed crystals containing several different molecular entities and moreover, their compositions evolve as the crystallisation proceeds. In this respect, the fractional crystallisation of fats differs fundamentally from other industrial crystallisation processes as used for instance for p-xylene, terephthalic acid, sugar, citric acid, etc. These processes are primarily purification processes that aim at the formation of pure crystals. Another factor complicating fat crystallisation is that fat crystals can have different morphologies and the crystallisation conditions must be such that only a single polymorph is formed. In addition, oils and fats—and this is particularly true for palm oil—invariably contain partial glycerides such as diacylglycerols that affect crystal growth, which may attach themselves to a growth site on the crystal and temporarily hinder the attachment of further triglyceride entities.
U.S. Pat. No. 5,874,599 discloses a process for the crystallisation of polymorphic fat molecules in a pseudo-steady state process, wherein the crystallisation is performed in a dry fractionation system by selecting and adjusting the flow rate, shear rate and temperature in such a way that the crystal form of the product is a kinetically-stable crystal form, while during the crystallisation a σ-value is maintained below 0.5, during a period of at least 12 hrs, wherein: σ=1−Sc/SE, where Sc is the percentage of solids in the crystalliser at the crystallisation temperature and SE is the percentage after stabilisation for 48 hours at the exit temperature of the crystalliser. The process uses a single crystalliser in which the crystallisation degree is close to equilibrium (solubility). When palm olein was used as starting material, the fractionation process yielded about equal amounts of top and bottom fractions and could be continued for 60-70 hours without giving rise to problems of encrustation, slurry stability, polymorphic form or viscosity.
U.S. Pat. No. 6,383,456 discloses an apparatus for fractionating a melt of mixed triglycerides, said apparatus comprising: a heat exchanger for supercooling the melt of mixed triglycerides; a nucleator for controlling the energy and condition of the melt of mixed triglycerides, said nucleator having an inlet and an outlet and including an agitator means, said inlet of said nucleator being connected to said heat exchanger; and a crystallizer connected to the outlet of said nucleator. In this process the nucleation stage is separated from the crystal growth stage. The examples in U.S. Pat. No. 6,383,456 are not limited to anhydrous milk fat, but include lard, tallow and palm kernel oil but do not include the fractionation of palm oil or its fractions.
EP 1 818 088A discloses a dry fractionation process for edible oils and fats comprising the steps of: melting the oil or fat to be fractionated; cooling the molten oil or fat in a crystalliser comprising a crystallisation vessel, an agitator or agitator assembly and a drive, thereby generating a slurry of crystals in a mother liquor; and subsequently separating said crystals from said mother liquor, whereby said drive provides said agitator or agitator assembly with an oscillating motion and/or a rotating motion around an axis, with the proviso that each point of said agitator or agitator assembly moves at substantially the same linear speed and in its examples describes the continuous fractionation of palm oil. Moreover, the gentle agitation intrinsic to this process surprisingly results in the formation of crystals of near uniform size, whereas in standard crystallisers, which comprise an agitator consisting of a rotating shaft with radially extending blades, several distinct populations of crystals having different sizes are obtained. This means that secondary nucleation of the crystallising melt is suppressed i.e., that no or hardly any nuclei are formed once the original nuclei started growing. Furthermore, EP 1 818 088A discloses that, contrary to what had been generally accepted, temperature homogeneity within a crystalliser is not a prerequisite for the formation of easily filterable crystals. The temperature gradient observed in Example 3 of EP 1 818 088A is such that it permits continuous operation.
A possible set-up of a continuous dry fractionation process shown in FIG. 6 of EP 1 818 088A in which there are three crystallisers in series each of which exhibiting a temperature gradient. Accordingly, the first crystalliser is fed with molten fat, and a crystal slurry ready to be filtered leaves the third one. Since the type of agitator used hardly causes vertical movement of the slurry, this set-up approaches a plug flow situation.
However, operating such a continuous dry fractionation process over an extended period of time will inevitably lead to an encrustation of solidified fat on the cooling elements used in the process, because the heat exchange surface must be markedly colder than the oil to achieve heat transfer. This causes their cooling capacity to decrease. Ultimately, the encrustation will be such that the cooling capacity will be insufficient and necessitate the interruption of the fractionation process to remove the solidified fat from said cooling elements. Therefore, the set-up of three crystallizers in series depicted in FIG. 6 of EP 1 818 088A permits in practice only a semi-continuous dry fractionation.
A plug flow is also aimed for in the continuous dewaxing process of vegetable oils and the same encrustation is also observed in this process. Oils like sunflower seed oil can contain variable amounts of waxes (esters between fatty acids and fatty alcohols) some of which have melting points above 70° C. These can cause the oil to become cloudy on cooling and since this is deemed to be undesirable, the high-melting waxes are removed by cooling the oil, thereby allowing these waxes to crystallise so that they can be removed by filtration. In comparison with the dry fractionation of edible oils and fats, the dewaxing process is quite simple. The molecules to be crystallised are less complex; they are quite different from the solvent (triglyceride oil) and to facilitate filtration, a filter aid is invariably used.
In continuous dewaxing such a plug flow is generally realised by using a crystalliser that is compartmented. Warm oil with dissolved waxes is fed at the top and oil with wax crystals leaves the vessel at the bottom and the temperature profile along the vessel remains the same. However, such vessels tend to suffer from encrustation of the cooling coils in the bottom of the vessel by wax deposits. These deposits decrease heat transfer and shift the cooling load towards the top of the vessel. This causes the oil in the top compartment to become so cold that freshly added oil is strongly undercooled so that many small wax crystals are formed. These require more filter aid than wax crystals that have been formed by slowly cooling the oil from above its cloud point to below its cloud point. Encrustation of cooling coils should therefore be avoided.